Navigate to the Test Coverage primary tab, and then to the Builds secondary tab. For the specific build, there will either be a Start a Test Session button if this will be the build's first test session, or there will be a New Test Session button if there has been at least one previous test session.
To start a test session, click the respective button to start the test execution. Note that test sessions can only be started if they are released for testing first.
Users will need view and edit permissions in the test coverage module to perform this action. Depending on the settings of a users project permissions, users may be able to view all project builds or just a subset of them. Project owners manage the permissions for project members.